1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method which form a toner image by using a liquid developer, and more particularly to an image forming apparatus and an image forming method which superimpose toner images of a plurality of colors on an image supporting member to obtain a color image.
2. Description of the Related Art
In an image forming system which obtains a toner image by an electrophotographic scheme using a liquid developer including toner particles and a carrier liquid, it is possible to use very fine toner particles of a submicron size as compared with, e.g., a one-component development mode (non-liquid). Using fine toner particles provides advantages of, e.g., an improvement in an image quality or realization of a texture comparable to printing (e.g., offset printing). Further, this image forming mode is cost-effective since a sufficient image concentration can be obtained with a small amount of toner, and is an energy-saving type because it can perform fixing (of toner) on a paper sheet at a relatively low temperature.
In case of obtaining a color image by an image forming apparatus which is of such an electrophotographic type using a liquid developer, there is a method which superimposes toner images of a plurality of colors on a photoconductor to obtain a color image and then collectively transfers the color image on the photoconductor onto a transfer target material. The collective transfer method has an advantage of a high positioning accuracy between colors and acquisition of a high-quality image. On the other hand, in the collective transfer method, since an image forming step is repeated on a photoconductor having a toner image, it is known that reproducibility of a image dot tends to be deteriorated at the image forming step of second and subsequent colors as compared with an image of a first color.
It can be considered that this phenomenon occurs because a charge imparting agent such as a metallic soap contained in a carrier liquid used in a liquid developer serves as an ion component (a counter ion) to be usually trapped and present in toner particles (including a “coloring agent”), but the charge imparting agent partly dissociates itself from the toner particles to freely behave mainly as a free phase ion.
As a method of removing such a free phase ion, removing an excess carrier liquid on a photoconductor before an image forming step of second and subsequent colors is proposed in Jpn. Pat. Appln. KOKAI publication No. 2002-108107 (p. 4, FIG. 1). That is, a method of removing a free phase ion described in the above-described publication uses a squeeze roller to decrease a carrier liquid contained in a precedent toner image having developed image dots on a photoconductive drum surface, and utilizes a drying device to lower a level of residual of the carrier liquid, thereby reducing the free phase ion.
However, in the method disclosed in the above-described publication, if adherence of toner particles of a preceding toner image with respect to a photoconductive surface is weak in a step of removing the free phase ion, the toner image (precedently formed on the photoconductive drum) may be possibly distorted (the toner particles may be possibly moved).
In order to avoid such distortion of a precedent toner image, for example, Jpn. Pat. Appln. KOKAI publication No. 2003-341290 proposes a method which allows toner particles to adhere to a photoconductive surface by set charging from an charger for the purpose of increasing adherence of the toner particles of a preceding toner image with respect to the photoconductive surface after end of development of the preceding toner image and before an image forming step of second and subsequent colors.
However, in the above-described method using set charging, a charger requiring a high voltage must be newly provided. This is nothing more than an obstacle for energy saving. Furthermore, in a color image forming apparatus in which a plurality of image forming units are provided around a photoconductor and toner images of a plurality of colors are superimposed during one revolution of the photoconductor, a charger for set charging is required for each image forming unit, and hence an installation space for each charger is necessary, which of course results in an increase in size.